If you don't know what to 'do with yourself down there' then you are most definitely not ready to have sex. Plus, you're only 13. You need to wait. And also, is the 'really nice boy you know' your boyfriend or just a random guy you want to sleep with? Get real! You are SO not ready for this...
First of all, you had sex with him while he was still kind of with her? Where is your self respect??? And how old are you? You sound like you're no more than 16 or so.
Second, you need to back the hell off from that guy. He still clearly has some kind of feelings for that girl, otherwise they...